There has been a growing interest in the investigation of high-k dielectric materials for reducing the operating voltages required for new flexible/printed electronics technologies. For example, in the case of organic field effect transistors (OFETs), while devices based upon conventional SiO2 dielectrics (k≈4, dielectric thickness ˜200-300 nm) typically operate at voltages between |30|-|50| V, and devices based upon conventional organic dielectrics (k<10) typically operate at voltages between |50|-|100| V, recent research has shown that OFETs with a high-k (e.g., k>10) gate dielectric can operate at much lower voltages (≤|40| V).
Various metal oxides with high dielectric constants have been investigated as an alternative to SiO2 gate dielectrics in OFETs. However, most of these metal oxides are based on ceramics, which require generally expensive deposition equipment and high-temperature annealing processes. Furthermore, these films can crack upon multiple bending because of their poor mechanical properties. Polymeric dielectrics offer good processability, but conventional dielectric polymers typically have low dielectric constants. Nevertheless, there are a few classes of polymers that are known to be high-k (k>10) polymers. One such class of polymers is poly(vinylidene fluoride) (PVDF) and its copolymers. Specifically, the dielectric constants of state-of-the-art PVDF-based polymers (e.g., poly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE); poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene), P(VDF-TrFE-CFE); and poly(vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene), P(VDF-TFE-HFP)) have been reported to be 20 or higher, and in some cases, 40 or higher, depending on the comonomers, backbone regiochemistry, and the ratio of the different repeating units. Their applicability in low-voltage devices has been confirmed by incorporating such PVDF-based polymers as gate dielectrics in OFETs.
To improve the electronic and mechanical properties of polymeric dielectrics and to integrate them into conventional device fabrication processes, it is desirable that these polymers can be crosslinked to achieve far more robust films. Even more desirable is if these polymers or their composite can be photocrosslinked (instead of crosslinked thermally or chemically), such that they can be photopatterned (i.e., patterned via photolithography) directly into patterned gate dielectrics without using a photoresist layer. For example, while peroxide/co-agent systems have been used to crosslink fluoropolymers, peroxide-based curing systems require thermal activation, and given that heat cannot be easily confined, peroxides and other heat-activated curing systems are not useful for patterning thin films where good resolution is critical. In addition, to be peroxide-curable, the fluoropolymer typically needs to incorporate a bromine- or iodine-containing monomer. Further, crosslinking reactions involving peroxides are known to form unwanted ionic species as side products, which if remained in the crosslinked network, will cause leakage current-related problems (breakdown, current-voltage hysteresis, bias stress) and thus are detrimental to the performance of electronic devices.
Past approaches of making PVDF-based polymers photocrosslinkable have focused mainly on incorporating a photocrosslinkable repeating unit into the polymer and/or the addition of bis-azides (e.g., 2,6-bis(4-azidebenzylidene)-4-methylcyclohexanone and other analogous compounds) as a photosensitive crosslinker in the polymer spin-coat solution. See e.g., International Publication Nos. WO2005064705, WO2013087500, and WO2013/087501, and US Patent Publication No. US20070166838.
Accordingly, there is a desire in the art to develop new methods of conferring photocrosslinkability to PVDF-based polymers and to provide photopatternable compositions including PVDF-based polymers, particularly, whereby thin films prepared from such compositions can be effectively photocrosslinked to withstand development with various organic solvents to provide patterned thin film components with high-k properties.